The development of disc driving apparatus utilized in the information processing field demands ever increasingly thinner dimensions and higher storage capacity. Specifically, higher performance, lower noise, longer life and higher shock-resistance are required. These requirements entail the same demands of motors that are to be employed in the disc driving apparatus for driving discs.
The bearing of a motor is one of the key elements that determine the above-identified specifications. Most of the conventional magnetic disc driving apparatuses have employed ball bearings in their motors, where discs of 5.25", 3.5", 2.5" or 1.8" diameters are mounted.
However, the technology of the ball bearing cannot meet the above demands any longer, and thus attention is drawn to a hydrodynamic bearing. Recently, the market has developed a demand for a thinner disc driving apparatus having a longer life span. Thus the employment of the hydrodynamic bearing is further extensively required.
The trend toward the hydrodynamic bearing and away from the ball bearing is described herein with reference to the magnetic disc driving apparatus (hereinafter called "apparatus".)
Because of the advancement of multi-media technology handling audio data and picture data, the apparatus must have a higher capacity among other things. The higher capacity requirement entails the higher recording density in the radius direction of the apparatus. The apparatus thus must accommodate a narrower pitch, which requires a lower NRRO (Non Repeatable Runout) of both a motor-hub spinning with a disc and the disc fixed to the hub.
The NRRO is an irregular shaft-run-out among run-outs of a rotary shaft. Two kinds of run-outs are available. One synchronizes with the shaft rotation, and the other does not synchronize. The NRRO is an unsynchronized run-out. The NRRO should be thus reduced in order to avoid producing errors in read and write operations. A specific NRRO demand of both the hub and disc has been approximately 0.4 .mu.m in a radial direction. However, 0.2 .mu.m is demanded recently because narrower track pitches are required due to higher storage capacities. Further, a demand of not more than 0.05 .mu.m is predicted in the future in order to accommodate the higher capacities.
It is well known that the NRRO of a conventional apparatus depends on the ball bearing that is one of the key parts of a disc driving motor. The ball bearing comprises an outer ring, inner ring, balls, holder, seal and grease.
A great dispersion on the NRRO can be produced by the following factors: (1) mechanical accuracy of a ball bearing, (2) a method of pre-loading the ball bearing, (3) control of the pre-loading thereof, and (4) assembling accuracy of the ball bearing into a motor. Among others, the following factors greatly influence the NRRO: mechanical accuracy of outer ring and inner ring, ball sphericity, and relative error of ball outlook when the ball bearing is incorporated into the motor. It is thus difficult to reduce the NRRO when the motor has been completely assembled.
Noises such as lathe noise are produced in the ball bearing by rolling on a track ring, and noise is produced due to self-induced vibration of a holder by itself. Further, a number of rotations of the motor are increased from 3600 rpm to 7200 rpm because the market demands a faster speed. As a matter of fact, a motor of 10000 rpm will a debut on the market. As such, the faster rotational speed of the motor produces larger noises, and the market demands lowering of the noises. In fact, the technology of the ball bearing hardly satisfies these demands.
According to a recent market trend, portable note-type personal computers have become disseminated in the market, and the apparatus therein employs a removable medium. This market trend requires that the motor be stronger against shock and drop. However, noise becomes greater when ca. 100 G acceleration due to shock or drop is applied to the ball bearing, because Brinell indentations are produced on a lathe face of the outer ring or inner ring. The technology of the ball bearing thus hardly meets the market demands, namely, a specific shock resistive value which is not less than 200 G.
The apparatus seals the entire motor assembly in a housing where the discs are mounted in order to always keep the apparatus clean. Because discs and magnetic heads are placed with extremely narrow spaces in between, when dust enters the space or hits either the disc or the head, the operation of read/write data is adversely affected. Therefore, the apparatus must be shielded from dust as mentioned above.
The space between the disc and head is presently ca. 0.1 m, however, it is necessary to reduce the space due to the demand for higher capacities. The construction of the motor does not allow the ball bearing to be lubricated, thus a grease-sealed type ball bearing is employed. The grease comprises base oil and thickener, both of which receive a shearing force due to spinning and start separating. The base oil and thickener spread to the discs, which damages the functions of the apparatus and resultant defects occur.
The above problems are caused by the ball bearings that the conventional motors employ. Therefore, employment of a hydrodynamic bearing in the motor can solve the above problems.
The hydrodynamic bearing comprises a cylindrical motor shaft and a hollow tubular sleeve in which the motor shaft is inserted with an annular clearance. On either the motor shaft or the sleeve, a plurality of herringbone-grooves are provided, and in the annular clearance or space between the shaft and sleeve is filled with lubricating fluid (oil or grease is often used). Driving of the rotor produces a pumping operation between the herringbone grooves and the lubricating fluid so that a dynamic pressure is generated in the radial direction. The dynamic pressure centers the shaft in the bearing, so that the rotating shaft is supported within the sleeve or the rotating sleeve is supported by the shaft, in a contact-free manner. On the other hand, herringbone grooves or spiral grooves are provided on at least either one of a thrust face of a thrust ring fixed to the shaft or a thrust face of the sleeve. The lubricating fluid is poured into the space between both of the thrust faces of the thrust ring and the sleeve, and thus dynamic pressure supporting the thrust load is generated axially. As a result, the rotating sleeve is supported by the thrust ring fixed to the shaft in a contactfree manner.
Recently, there has been a desire to produce a thinner magnetic disc driving apparatus which has a longer life. When the apparatus becomes thinner, the total height of the motor must be thinner because the height of the motor is restricted by the dimensions of the apparatus. An axial length of the sleeve, which lowers rigidity of the bearing, is difficult to obtain because the sleeve is a key element of the hydrodynamic bearing. As a result, mechanical contact is produced between the shaft and sleeve as well as between the sleeve thrust faces and the fixed ring thrust face due to starts and stops and overloading of the motor. These mechanical contacts cause abrasion and seizure at a sliding region between the shaft, which is made of stainless alloy, and the sleeve, which is made of copper system alloy, as well as between both of the thrust faces of the sleeve and the fixed thrust ring.
In the case of laser beam printers, a motor employed therein rotates only when letters or data are printed. The motor thus operates intermittently. Further, a function of power savings is incorporated into the motor, which halts the motor to eliminate consumption of electric power when the printer is not operated. As such, it has become a general trend for the motor to be used under a condition of repeating starts and stops.
When the laser beam printer becomes thinner in dimension, the height of motor is restricted to within the dimension of the printer, and therefore, the total height of motor must be lower. Then it is difficult to obtain an axial length of a sleeve, and therefore a bearing rigidity becomes insufficient.
The insufficient rigidity of the bearing entails mechanical contacts between a shaft and the bearing due to starts and stops, as well as over loading of the motor. As a result, abrasion and seizure are produced on a sliding region between the motor shaft made of stainless alloy and the bearing made of copper alloy.
Various proposals have been developed in an attempt to solve the above problems. U.S. Pat. No. 4,652,149 is one of the proposed solutions. This patent discloses the following advantages: a partial or entire bearing comprises self-lubricating resin such as polyacetal, nylon or the like, and this resin includes carbon fiber in a predetermined quantity so that electric conductivity is imparted to the resin. A motor shaft is plated with chromium (Cr), and thus the conductivity between the bearing and the motor shaft can be checked. This enables users to check whether metallic powder, which is harmful with regard to the width of clearance, exists between the motor shaft and the bearing, or whether contact exists between a fixed side and a rotating side.
However, since this bearing is formed partially or entirely by the self-lubricating resin, its hardness is lower than that of a regular bearing made of metal. The bearing disclosed by this U.S. Patent undergoes more abrasion than that experienced with the regular bearing. When the resin bearing is employed in a motor that rotates at a higher speed or undergoes frequent start and stop repetitions, a significant amount of abrasion occurs in the resin section. Therefore, this bearing is not recommended in motors which are employed in disc driving apparatuses and laser beam printers because of the life span of the bearing.
Another proposal is disclosed in U.S. Pat. No. 4,838,710. This patent discloses a static gas bearing which includes a gaseous fluid discharging member that supports a shaft. In other words, the bearing member is made of porous graphite, and the shaft surface is processed by ion plating and coated by titanium nitride so that the quality of the sliding surface of the shaft is improved.
Japanese Patent Application non-examined Publication No. S59-89823 also discloses some measure of protection against abrasion which includes a contacting face of a bearing that is coated by hard composite material such as titanium carbon and tungsten carbon through the ion plating method in order to provide abrasion-resistance to the bearing, which prevents the bearing from being worn.
In addition, the U.S. Pat. No. 4,555,186 discloses an improved bearing which includes a thin film made of amorphous alloy formed on a hollow cylindrical bearing. The Vickers hardness of the thin film is set to be not less than 1000.
As disclosed in U.S. Pat. No. 4,838,710 and Japanese Patent Application non-examined Publication No. S59-89823, the surfaces of the bearing and the sleeve are coated by hard material such as titanium carbon, tungsten carbon or titanium nitride through the ion plating method, thereby raising the hardness and improving the abrasion-resistance. However, the ion plating method entails temperatures rising up to ca. 300.degree. C. in the process atmosphere. Materials having low fusing points such as the lead (Pb) included in the copper system alloy, which is a material of the sleeve, are deposited in this atmosphere. This deposition causes a failure of close contact between the coating layer and the surfaces as well as dispersion of a film thickness. This dispersion results in a reduced degree of precision with respect to the inner diameter of the sleeve. The ion plating method requires large equipment including a vacuum furnace, and further, this method takes well over half an hour to complete the process. Low productivity is, therefore, a disadvantage of this method.
The surface hardness of titanium carbon or tungsten carbon exceeds 2000 Vickers hardness. The hardness of the thin film made of amorphous alloy exceeds 1000 Vickers hardness as disclosed in U.S. Pat. No. 4,555,186. However, when the hardness of a bearing surface exceeds 1000 Vickers hardness, it is significantly harder than the surface hardness range of a shaft and a fixed thrust ring, i.e. 700.+-.100 Vickers hardness. Thus, when the bearing of this combination operates for a long time, the shaft and fixed thrust ring will wear out.
Japanese Patent Application non-examined Publication No. S61-112818 is directed to improved abrasion resistance and discloses a boron (B) composite electroless nickel plating which is provided on the contacting surface of a bearing, thereby providing abrasion-resistance to the surface. It is preferable that a thickness of the plating ranges from 10 to 20 m and the surface has ca. 600 Vickers hardness.
However, the boron composite electroless nickel plating uses boron chloride as a reducing agent, and the film comprises Ni, P and B. This plating is expensive, and its plating bath has a problem with regard to stability. Further, when plating becomes thicker, a width of the herringbone grooves formed inside of the sleeve becomes narrower, which produces less rigidity of the shaft than a predetermined rigidity. The thickness of plating is proportional to plating time, and therefore, a thicker plating lowers productivity. It is concluded that this boron composite electroless nickel plating is not practical in a productivity-oriented manufacturing operation.